In order to improve the performance of the mold, many manufacturers will properly process their molds. Mold processing refers to the processing of forming and billet tools, and also includes shearing dies and die-cutting dies. It will also reflect processing defects, resulting in a decline in mold performance, so how to build mold processing defects? The following seven measures can solve the defects of mold processing.
1. Reasonable selection and dressing of grinding wheels
The grinding wheel using white corundum is better, its performance is hard and brittle, and it is easy to produce new cutting edges, so the cutting force is small, the grinding heat is small, and the medium particle size is used in the particle size, such as 46-60 mesh is better. The hardness of the grinding wheel is medium-soft and soft (ZR1, ZR2 and R1, R2), that is, coarse-grained, low-hardness grinding wheels, which have good self-excitation and can reduce cutting heat. It is very important to choose the appropriate grinding wheel for fine grinding. For the high vanadium and high molybdenum conditions of the mold steel, it is more suitable to choose GD single crystal corundum grinding wheel. When processing cemented carbide and materials with high quenching hardness, diamond with organic binder is preferred. Grinding wheel, organic binder grinding wheel has good self-grinding property, and the roughness of the workpiece can reach Ra0.2μm. In recent years, with the application of new materials, CBN (cubic boron nitride) grinding wheel has shown a very good processing effect. Finishing on CNC forming grinders, coordinate grinders, and CNC internal and external cylindrical grinders, the effect is better than other types of grinding wheels. During the grinding process, it is necessary to pay attention to dressing the grinding wheel in time to keep the grinding wheel sharp. When the grinding wheel is passivated, it will slide and squeeze on the surface of the workpiece, causing burns on the surface of the workpiece and reducing its strength.
2. Rational use of cooling lubricant
Play the three major functions of cooling, washing and lubrication, keep the cooling and lubrication clean, so as to control the grinding heat within the allowable range, so as to prevent thermal deformation of the workpiece. Improve the cooling conditions during grinding, such as using oil-immersed grinding wheels or internal cooling grinding wheels. The cutting fluid is introduced into the center of the grinding wheel, and the cutting fluid can directly enter the grinding area to exert an effective cooling effect and prevent the surface of the workpiece from being burned.
3. Reduce the quenching stress after heat treatment to a minimum
Because of the quenching stress and the network carbonized structure under the action of the grinding force, the phase transformation of the structure can easily cause cracks in the workpiece. For high-precision molds, in order to eliminate the residual stress of grinding, low-temperature aging treatment should be carried out after grinding to improve toughness.
The vacuum heat treatment of the mold includes preliminary heat treatment, final heat treatment and surface strengthening treatment. Generally, heat treatment defects refer to various defects that occur during the final heat treatment process of the mold or in the subsequent process and during use, such as quenching cracks, out-of-tolerance deformation, insufficient hardness, electrical machining cracks, grinding cracks, and early damage to the mold wait. Let's take a closer look at these defect prevention measures with the editor! picture
Quenching
The causes and preventive measures of quenching cracking are as follows:
1. The shape effect is mainly caused by design factors, such as the fillet R is too small, the hole position is not set properly, and the section transition is not good.
2. Overheating (overburning) is mainly caused by inaccurate temperature control or running temperature, irregular and unreasonable vacuum heat treatment process, especially insufficient tempering. The setting temperature is too high, the furnace temperature is uneven and other factors are caused. The preventive measures include maintenance, proofreading the temperature control system, correcting the process temperature, and adding shim iron between the workpiece and the furnace floor.
3. Decarburization is mainly caused by factors such as overheating (or overburning), unprotected heating in an air furnace, small machining allowance, residual decarburization layer in forging or preliminary heat treatment, etc. The preventive measures are controlled atmosphere heating, salt bath heating, Vacuum furnaces and box furnaces are protected by boxing or anti-oxidation coatings; the machining allowance is increased by 2 to 3mm.
4. Improper cooling is mainly caused by improper selection of coolant or overcooling. It is necessary to master the cooling characteristics of the quenching medium or tempering treatment.
5. The organization of raw materials is poor, such as serious carbide segregation, poor forging quality, improper preparatory heat treatment methods, etc. The preventive measures are to adopt correct forging process and reasonable preparatory heat treatment system.
Insufficient hardness
The reasons and preventive measures for insufficient hardness are as follows:
1. The quenching temperature is too low, mainly due to improper process setting temperature, temperature control system error, improper furnace loading or entering the cooling tank, etc., the process temperature should be corrected, the temperature control system should be overhauled, and the workpiece interval should be adjusted during furnace loading. Arrange them reasonably and evenly, disperse them into the tank, and prohibit stacking or bundling them into the tank for cooling.
2. The quenching temperature is too high, which is caused by improper process setting temperature or temperature control system error. The process temperature should be corrected, and the temperature control system should be overhauled and checked.
3. Over-tempering, which is caused by setting the tempering temperature too high, fault error of the temperature control system or entering the furnace when the furnace temperature is too high. The process temperature should be corrected, and the temperature control system should be overhauled. enter.
4. Improper cooling, the reason is that the pre-cooling time is too long, the cooling medium is not properly selected, the temperature of the quenching medium is gradually high and the cooling performance is reduced, the stirring is not good or the temperature of the tank is too high, etc. Measures: out of the furnace, enter the tank, etc. quickly; master the quenching medium Cooling characteristics: oil temperature is 60-80°C, water temperature is below 30°C, when the quenching amount is large and the cooling medium heats up, cooling quenching medium should be added or other cooling tanks should be used for cooling; the stirring of the coolant should be strengthened; at Ms+50°C when removed.
5. Decarburization, which is caused by the residual decarburization layer of raw materials or quenching and heating. The preventive measures are controlled atmosphere heating, salt bath heating, vacuum furnaces and box furnaces are protected by packing or using anti-oxidation coatings; Increase the amount by 2 to 3mm.
Out of tolerance
In mechanical manufacturing, the quenching deformation of heat treatment is absolute, while non-deformation is relative. In other words, it's just a matter of deformation size. This is mainly due to the surface relief effect of the martensitic transformation during heat treatment. Preventing heat treatment deformation (dimensional changes and shape changes) is a very difficult task and in many cases has to be solved empirically. This is because not only the steel type and mold shape have an impact on heat treatment deformation, but also improper carbide distribution and forging and heat treatment methods will also cause or aggravate it, and in many heat treatment conditions, as long as a certain condition changes, the deformation of steel parts The degree will vary greatly. Although the problem of heat treatment deformation has been mainly solved by experience and trial methods for a long time, it is necessary to correctly grasp the relationship between raw material forging, module orientation, mold shape, heat treatment method and heat treatment deformation, and grasp the law of heat treatment deformation from the accumulated actual data. However, it is a very meaningful work to establish archives about heat treatment deformation.
decarbonization
Decarburization is a phenomenon and reaction in which all or part of the carbon on the surface layer is lost due to the effect of the surrounding atmosphere when the steel part is heated or kept warm. Decarburization of steel parts will not only cause insufficient hardness, quenching cracks, heat treatment deformation and chemical heat treatment defects, but also have a great impact on fatigue strength, wear resistance and mold performance.
Cracks caused by electrical discharge machining
In mold manufacturing, the use of electrical discharge machining (electric pulse and wire cutting) is more and more commonly used processing methods, but with the wide application of electrical discharge machining, the defects caused by it also increase accordingly. Since electrical discharge machining is a processing method that melts the surface of the mold by means of the high temperature generated by electrical discharge, a white electrical discharge machining metamorphic layer is formed on the processed surface, and a tensile stress of about 800 MPa is generated. In this way, during the electrical machining process of the mold Defects such as deformation or cracks often appear in the middle. Therefore, it is necessary to fully grasp the influence of electric discharge machining on the mold material, and take corresponding preventive measures in advance. Prevent overheating and decarburization during heat treatment, and perform sufficient tempering to reduce or eliminate residual stress; in order to fully eliminate internal stress generated during quenching, high-temperature tempering is required, so steel types that can withstand high-temperature tempering should be used (such as Crl2 type, ASP-23, high-speed steel, etc.), process under stable discharge conditions; after discharge machining, perform stabilization and relaxation treatment; set reasonable process holes and grooves; fully eliminate the re-solidified layer, so that in a sound state Next use; using the principle of vector translation, the part of the internal stress that has been concentrated in the cutting sentinel is released through drainage.
Insufficient toughness
The reason for the lack of toughness may be that the quenching temperature is too high and the holding time is too long to cause grain coarsening, or because tempering is not avoided in the tempering brittle zone.
grinding crack
When there is a large amount of retained austenite in the workpiece, under the action of grinding heat, tempering transformation occurs, resulting in structural stress and cracking of the workpiece. The preventive measures are: cryogenic treatment or repeated tempering after quenching (die tempering is generally 2 to 3 times, even for low alloy tool steel for cold working), to minimize the amount of retained austenite.
